@Philipp_Ossler on 7.6 this does not appear to be the behaviour i am seeing.
var correlation = execution.getProcessEngineServices()
.createMessageCorrelation('response_' + myIdVar)
When this executes, I have a another process that is waiting, but there are two instances (in different process def versions) waiting for that message.
the above code gets the following result:
org.camunda.bpm.engine.MismatchingMessageCorrelationException: ENGINE-13031 Cannot correlate a message with name 'response_181' to a single execution. 2 executions match the correlation keys: CorrelationSet [businessKey=null, processInstanceId=null, processDefinitionId=null, correlationKeys=null, tenantId=null, isTenantIdSet=false]
There is only one process instance in the latest process def version that is waiting for this message.
Based on your description, this should not be occurring.